// 计算属性名
var pre='foo';
var myobj={
    [pre+"bar"]:"hello",
    [pre+"baz"]:"world",
    [Symbol.something]:'hello world'
}
console.log(myobj["foobar"]);
console.log(myobj["foobaz"]);
// 给数组添加属性
var myArray=['foo','42','bar'];
myArray.baz="baz";
myArray.length=3;
console.log(myArray.baz);// baz